1. Field of Invention
The present invention pertains to capacitors and the dielectrics employed in their construction.
2. Description of Prior Art
A basic inner capacitor structure consists of one pair of plates, separated by a dielectric. Most practical capacitors consist of an array of pairs of facing plate surface areas, the members of each facing pair being separated one from another by a dielectric.
The prior art has attached great significance to this dielectric that is situated between facing pairs of plates. It has been recognized that this dielectric affects the performance of the capacitor, because the displacement current and electrical charge/discharge occurs through this dielectric.
The art recognizes that protective or housing dielectrics, partially or wholly encapsulating the basic inner capacitor structure, are useful to convert a basic inner capacitor structure into a housed capacitor, to make a finished capacitor product having utility in the real world. However, prior art has not attached great significance to any dielectric that is situated outside the space between facing pairs of plates, including protective or encapsulating dielectrics. Prior art has operated under the theory and belief that all of the displacement current and electrical charge/discharge between facing pairs of plates occurs only through the dielectric that is situated between facing pairs of plates, and that therefore any dielectric situated outside this space cannot substantially affect the electrical performance of a capacitor.
Thus, prior art capacitors aiming for excellent performance typically employ a dielectric in the space between facing pairs of plates that is excellent, and excels in a recognized measurable parameter such as low dissipation factor; examples are flexible plastic film dielectric materials such as polypropylene, polystyrene, or tetrafluoroethylene. However, when making a finished capacitor product suitably protected by encapsulation for utility in the real world, prior art has employed an encapsulating dielectric material outside this space that is inferior in the same recognized parameter, for example having a higher dissipation factor. Such prior art finished capacitor products usually employ polyester tape as the outer wrapping and epoxy as the end fill of a wrap and fill roll capacitor configuration, or surround the plate array on all sides with epoxy in an encapsulated configuration. Polyester and epoxy are substantially inferior dielectric materials, and measure as having substantially higher dissipation factor, compared to polypropylene, polystyrene, or tetrafluoroethylene. Prior art finished capacitor products have intentionally employed these inferior protective or encapsulating dielectric materials, outside the space between facing pairs of plate surface areas, because these inferior dielectric materials can be cheaper, sturdier, and/or more volumetrically efficient. Prior art has not recognized any problems or electrical performance degradation from their employment of inferior dielectrics outside the space between facing pairs of plates.
A typical prior art film capacitor, for example one rated at 400 VDC, employs a plastic film of 12 micron thickness as a dielectric between facing pairs of plates. The predominant material of this plastic film for a high quality capacitor is typically polypropylene, which has a measured dissipation factor at 1 KHz of 0.0003.
Such a typical prior art film capacitor, as a finished capacitor product, has employed a different dielectric outside the space between facing pairs of plate surface areas. It has employed a protective or encapsulating outer wrap tape made of polyester and adhesive, applied for at least three circumferential revolutions around the capacitor. The thickness of this outer wrap tape has comprised 25 microns polyester, plus the adhesive thickness (which can more than double the thickness for this outer wrap tape). Three layers of this tape have had a thickness of 75 microns of polyester, plus adhesive. Thus, the thickness of the dielectric outside the space between facing pairs of plates has been 75/12, more than 6 times greater, than the thickness of the dielectric between facing pairs of plates--without even counting the added thickness of the adhesive of the outer wrap tape.
The measured dissipation factor for polyester at 1 KHz is 0.0047, which is over 15 times higher (worse) than that of polypropylene. The dissipation factor of common tape adhesives is probably even worse. Measured dissipation factor could be taken as a quality index of dielectric materials, where a quantitatively higher index indicates poorer dielectric quality. Thus, the material of this outer wrap tape has been over 15 times worse in dielectric quality (not even counting the adhesive) than the material between facing pairs of plates.
The thickness of this 15 times worse dielectric material has been more than 6 times greater than the better dielectric material employed between facing pairs of plates. The total degradative effect upon a electromagnetic wave in space (or upon a signal being carried by this wave) by an instance of an inferior dielectric could be taken to be given by the quality index of the dielectric material multiplied by the dimension (e.g. thickness) of that material through which the electromagnetic wave must travel instead of travelling through free space (which is not degradative)--where the quality index is some recognized measurement of dielectric material quality, mathematically configured such that a higher index indicates poorer dielectric quality (e.g. dissipation factor, dielectric constant, etc.). To compare the overall quality of instances two dielectrics, the product of quality index and thickness for one could be compared with a similar product for the other. Thus, the overall quality of the outer wrap dielectric employed by prior art high quality film capacitors has been more than 90 (15 times 6) times worse than the overall quality of the dielectric employed by them between facing pairs of plates.
Some lesser quality capacitors of prior art have employed polyester as the dielectric between facing pairs of plates, and also polyester for the outer wrap tape. Since the dielectric strength of polyester (7000 volts per mil) is over twice that of polypropylene (3000 volts per mil), the thickness of polyester that has been employed between facing pairs of plates has been typically less than the thickness employed of higher quality dielectric materials such as polypropylene. Meanwhile, the thickness of outer wrap tape has remained substantially the same. Thus in such capacitors the thickness of the dielectric outside the space between facing pairs of plates has typically been over twice times 6, i.e. over 12 times the thickness of the dielectric employed between facing pairs of plates. Even though the material is the same (still not counting the adhesive), the capacitor's outer wrap has had about 12 times more degradative effect than the dielectric between facing pairs of plates.
Furthermore, wrap and fill film finished capacitor products of prior art have employed a protective or encapsulating epoxy end fill. The dissipation factor of epoxy is 0.01, over 33 times worse than polypropylene. The typical thickness of this epoxy end fill has been about 3175 microns (1/8 inch), which is over 264 times thicker than 12 microns of the polypropylene film dielectric between facing pairs of plates. Thus, the overall quality of an instance of this dielectric employed by these high quality prior art capacitors outside the space between facing pairs of plates has been over 8712 times worse than the dielectric employed between facing pairs of plates.
Other film finished capacitor products of prior art have been totally encapsulated in epoxy, either by dipping or by potting within a hard plastic case. The thickness of the epoxy coating for such capacitors has encompassed a range of approximately 1000 to 10,000 microns (1 mm to 10 mm). Thus such prior art capacitors have employed a dielectric outside the space between facing pairs of plates whose overall quality is 2750 to 27,500 times worse than the dielectric between facing pairs of plates, where this latter dielectric is made from a high quality material such as polypropylene. The adverse impact of this inferior dielectric is compounded by the fact that it totally encases these prior art capacitors.
Other prior art capacitors (e.g. Wada, Wilheim) have employed other materials as the inner dielectric between facing pairs of plates, including rigid materials such as ceramic, glass, and printed circuit board. The art has recognized that it is necessary to encapsulate and protect the basic inner capacitor structure employing these various inner dielectrics, in order to make a finished capacitor product having utility in the real world. This is especially true for rigid dielectric materials, which are fragile in the thin gauges required between plates to obtain reasonably large capacitance. For this encapsulation, required to make a finished capacitor product, prior art has used a much greater (by at least 20 times) thickness of the same material as the inner dielectric, or has used a much thicker layer of a different material, usually with inferior dielectric quality. Some prior art patents have failed to specify the explicit materials and dimensions of the encapsulation required to make their basic inner capacitor structure into a finished capacitor product having utility in the real world, thereby assuming that conventional manners of encapsulation recognized by the art are acceptable to them. It can be appreciated that these prior art patents, by failing to explicitly address the issue of encapsulation, or by failing to explicitly specify the materials and dimensions of the encapsulation, necessarily fail to anticipate the present invention, and that the present invention is necessarily unobvious in the light of such prior art patents.
In sum, prior art finished capacitor products have employed a dielectric, outside the space between facing pairs of plates, whose overall quality ranges from 12 to 27,500 times worse than the dielectric they employ between facing pairs of plates. To be conservative, one could say that in prior art finished capacitor products the overall quality of the dielectric outside the space between facing pairs of plates has been more than 6 times worse than the overall quality of the dielectric between facing pairs of plates.
Prior art has not recognized that there is a problem with the dielectric outside the space between facing pairs of plates being significantly inferior to the dielectric between facing pairs of plates. As noted, prior art has operated under the theory and belief that all of the displacement current and electrical charge/discharge between facing pairs of plates occurs only through the dielectric that is situated between facing pairs of plates, and that therefore any dielectric situated outside this space cannot substantially affect the electrical performance of a capacitor.
However, it has now been experimentally determined and demonstrated that a dielectric, situated outside the space between facing pairs of plate surface areas, does substantially affect the electrical performance of a capacitor, and that the electrical performance of a capacitor can be improved when the overall quality of the dielectric outside the main plates, relative to the overall quality of the dielectric between the main plates, is improved over prior art.